What CD authoring program are you using? Did this just start happening, or has it always been like this? Did you upgrade the CDR drive or any software?

Although, you would think the authoring software would recognize the speed of the medium and adjust the output speed accordingly.

I had an issue once where my old Plextor CDRW drive recorded fine for months and then stopped. Nothing was changed either. I solved it by making the CDRW drive the master device and made the DVD the slave. How is it hooked up on your system?